In order to extract maximum amount of oil from the earth, at least one injection well and several production wells are typically drilled into the target oil formation (e.g., an oil reservoir). Typically, water is pumped into the injection well to displace low-viscosity oil from the target formation to the production well. After water-flooding the target formation, the residual oil saturation ranges from 30% to 50%, depending on the oil-water mobility ratio. To reduce the residual oil saturation to below 20%, steam, miscible gas or a surfactant solution is pumped into the injection well to flood the target formation to the production wells.
Because of their inherent high viscosities, tars and heavy oils can not be displaced at economic rates with water, surfactant solutions or miscible gas to the production well. As such, thermal oil recovery techniques are often used to reduce the in-situ oil viscosity to below 100 cp so the oil can be displaced by a fluid or gravity drained at economic rates. Currently available techniques include steam injection, in-situ combustion, and in-situ wet combustion.